1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer program product.
2. Description of the Related Art
In accordance with market demand, electrophotographic devices that can output color images, such as color copiers and color printers, are used more and more. Especially, with the demand for color-image outputting speeds as high as monochrome-image outputting speeds, tandem-type color-image forming apparatuses that include color-based photosensitive elements and color-based developing devices have recently become mainstream. The tandem-type color-image forming apparatuses form toner images with different single colors on individual photosensitive elements and then sequentially transfer the single-color toner images onto a transfer sheet, thereby recording a color image (see, for example, Japanese Patent Application Laid-open No. 2006-126643).
In a typical tandem-type color-image forming apparatus, regardless of whether it is a direct-transfer type or an intermediate-transfer type, the single-color images are transferred from the individual photosensitive elements, at different positions on an intermediate transfer belt, to the intermediate transfer belt or the transfer sheet; therefore, even a fine change in the moving speed of the intermediate transfer belt alters the timing at which the intermediate transfer belt reaches the transfer position of the next single-color image, which in turn causes the transfer positions of the single-color images to shift from the correct positions, which in turn results in an output image with misalignment (color shift) in the sub-scanning direction.
A typical tandem-type color-image forming apparatus includes writing units separated from each other on the basis of color. If the constituents are displaced from the correct positions due to an environmental change, such as a temperature change, which in turn changes the magnifying power and the writing position, an output image with misalignment in the main-scanning direction is formed.
A typical image forming apparatus that includes both a direct-transfer system and an indirect-transfer system calculates, using a sensor, the amounts of misalignment of positional alignment patterns that are formed on the intermediate transfer belt and adjusts the positions of the multi-color images to be formed by the indirect-transfer system in the same manner as adjustments are made in an indirect-type image forming apparatus. Moreover, an image forming apparatus that includes both a direct-transfer system and an indirect-transfer system needs to adjust the relation between the position of a black image that is formed by the direct-transfer system and the positions of multi-color images.
Therefore, as well as the positional-alignment sensor that is arranged on the intermediate transfer belt in the indirect-transfer system and is used in the adjustment of the positions of the multi-color images, another positional alignment sensor is also needed in the direct-transfer system. This other positional alignment sensor is used in the adjustment of the relation between the position of the image that is formed by the direct-transfer system and the position of the image that is formed by the indirect-transfer system. Because two sensors are included, the number of necessary parts increases, which results in a more complicated configuration and more complicated maintenance operations.
In some of the image forming apparatuses that include both the direct-transfer system and the indirect-transfer system, the image forming units and the intermediate transfer belt of the indirect-transfer system are designed so that they are detachable from the main body. Such an image forming apparatus can become a monochrome image forming apparatus when only the image forming unit of the direct-transfer system is operable. When such an image forming apparatus is used as a monochrome image forming apparatus, because a monochrome image is formed, the two positional-alignment sensors are unnecessary. From the design perspective, it is necessary for both the positional-alignment sensors and the image forming units of the indirect-transfer system to be detachable from the main body, which results in a complicated attachment/detachment operation.
Japanese Patent Application Laid-open No. 2006-126643 discloses a technology related to an image forming apparatus that can reduce the printing time in the monochrome mode and increase the accuracy in positional alignment for color printing. The image forming unit for black has an indirect-transfer mode and a direct-transfer mode. During color-image forming, the image forming unit for black is set to the indirect-transfer mode and the images of different colors including black are transferred onto the intermediate transfer medium in a superimposed manner.
However, in the image forming apparatus that includes both an indirect-transfer system and a direct-transfer system according to the technology disclosed in Japanese Patent Application Laid-open No. 2006-126643, the sensors are still needed in both the indirect-transfer system and the direct-transfer system.